1. Field of the Invention
The invention relates generally to an apparatus for retaining a workpiece on a workpiece support within a semiconductor wafer processing system and, more specifically, to an improved three piece wafer support assembly for retaining and temperature regulating large diameter (300 mm or more) semiconductor wafers.
2. Description of the Background Art
In semiconductor wafer processing equipment, electrostatic chucks are commonly used for clamping 200 millimeter (mm) wafers to a pedestal during processing. Electrostatic chucks typically clamp a workpiece (i.e., a semiconductor wafer) by creating an electrostatic attractive force between the wafer and the chuck. A voltage is applied to one or more electrodes in the chuck so as to induce oppositely polarized charges in the wafer and electrodes, respectively. The opposite charges pull the wafer against the chuck, thereby retaining the wafer. For example, in a physical vapor deposition (PVD) chamber a 200 mm wafer is electrostatically clamped to an electrostatic chuck disposed of a wafer support assembly, to ensure that the wafer is stationary and temperature regulated during processing.
Increased demand for 200 mm wafers led to improvements in chuck construction and features for processing this size workpiece. This resulted in higher wafer yield, better temperature control during wafer processing, and an overall better quality product. The latest generation of semiconductor wafers has a diameter of 300 mm, which accommodate fabrication of even more integrated circuit components on a single wafer. Unfortunately, the larger size wafers and smaller device dimensions carry with them their own set of production problems.
For example, wafer processing temperatures as low as −60° C. may be required. As such, a larger thermal transfer element (e.g., cooling plate) is required to provide adequate cooling of a 300 mm wafer during processing. Additionally, maintaining adequate and uniform thermal conductivity between the thermal transfer element and the backside of the wafer at any operating temperature is desirable. For example, during pre-wafer process bake-out of the chamber and electrostatic chuck (i.e., to remove excess moisture) the entire electrostatic chuck should be uniformly heated to completely remove moisture and any other potential contaminants.
One solution was to develop a two-piece assembly whereby the chuck and thermal transfer element are individual components and capable of operating at low processing temperatures. In two piece assemblies, the chuck portion resembles a disk-like portion and is commonly referred to as a puck. Usually the puck and thermal transfer element are fabricated from different materials. For example, the puck is fabricated from a ceramic puck (e.g., AlN), while the thermal transfer portion (i.e., cooling plate) is illustratively fabricated from molybdenum or molybdenum alloy, KOVAR®, or a metal matrix composite (AlxSiySiC). These materials are joined together by brazing. However, brazing temperatures cause thermal expansion to occur at the surface being brazed, which may result in deformation of the puck and cooling plate. For example, the support surface is designed to operate at temperatures in the range of −60° C. to 50° C., and a bake out process occurs in a temperature range of 100° C. to 350° C. As such, the bake out temperature range puts stringent conditions on the types of materials a manufacturer may use to build the electrostatic chuck assembly. In particular, conventional bonding techniques, such as using an Indium alloy, are not reliable in this temperature range due to a low melting point of 156° C. for indium.
Additionally, at extreme operating temperatures, differential thermal expansions of the wafer support assembly components occur. In particular, under thermal load, a material will change shape proportional to the amount of temperature change multiplied by its coefficient of thermal expansion. The coefficient of thermal expansion indicates how much a material shape will change for each degree of temperature change. Typically, a ceramic puck, such as aluminum nitride (AlN), has a thermal expansion coefficient of approximately 5×10−6 per degrees C., while stainless steel has a coefficient of thermal expansion of approximately 17×10−6 per degrees C. As such, the ceramic puck will expand approximately 3 times less as a similarly sized stainless steel part. When he aluminum nitride and stainless steel are joined together, such thermal expansion differentials may quickly lead to stress and cracking.
Another problem is in an instance where molybdenum is used to fabricate the cooling plate. In particular, molybdenum cannot be easily welded to a metal such as stainless steel, aluminum, and the like. Welding molybdenum to stainless steel requires the welding to be performed in a vacuum-like environment. As such, manufacturing difficulties arise when welding a molybdenum cooling plate to a stainless steel pedestal. Furthermore, welding at high temperatures may cause the molybdenum cooling plate to become brittle, thereby increasing susceptibility to fatigue and cracking. Moreover, contaminants may form and combine with the weld, thereby weakening the strength of the bond.
Therefore, there is a need in the art for a low processing temperature 300 mm puck and thermal transfer element assembly and a technique for securely joining the puck, cooling plate, and pedestal. Such devices are necessary to improve temperature uniformity across a wafer, maintain the wafer at specific temperature ranges during processing, and reduce the maintenance and manufacturing costs of the same.